There are a lot of other factors to consider, but how much it costs every time you pull the trigger is certainly important.

Why I did this- I was thinking about getting into 7.62x39mm shooting as a cost saving measure over 5.56 or .308, wanted to see if it was significantly less expensive. I was curious how much cheaper .22 Mag was over 5.7; also was interested if the .40 had caught up with 9mm in price. Also, looking around, it seemed 12 gauge was cheaper than .410, this didn't make sense, so I decided to do some online comparisons. These price per round (*estimates) do not include shipping or taxes.

Don't ask how I came up with these 'raw' figures, they will be useful to many especially new shooters; you gun guru's already know the deal.

Cheapest to most expensive (approximation of median price per round), of the calibers I picked to compare, not conclusive, not scientific.

Some rounds have a huge range in price from the cheapest commonly available to the most exclusive; here's some examples:
You can find 12 gauge shells from 21 - 320 cents per shell; and 357 SIG ranged from 29 - 181 cents per shell, huge differences. The smallest variances in pricing was for 22 LR (3-6 cents per round), 9mm (16-22 cents per round), 17HMR (20-28 cents per round) and 7.62x39mm (17-25 cents per round). I didn't take into account the absolute cheapest or most expensive rounds (no blanks, no special loads or exotics; just the stuff that most people routinely shoot).